1. Technical Field
The present subject matter relates to semiconductor memory, and more particularly, to compensating for an off-current of the memory.
2. Background Art
Many types of semiconductor memory are known in the art. One type of memory is flash memory which stores charge in a charge storage region of a memory cell. The voltage threshold of the metal-oxide-semiconductor field-effect transistor (MOSFET) based flash cell can be changed by changing the amount of charge stored in the charge storage region of the cell, and the voltage threshold can be used to indicate a value that is stored in the flash cell. By providing a voltage across the flash cell that is between the voltage thresholds of the two different states of the flash cell, the state of the flash cell can be determined by measuring current flowing through the flash cell. A flash cell has a much higher on-current than off-current.
Another type of memory is phase change memory (PCM). PCMs utilize a phase change material having a non-conductive amorphous state and a conductive crystalline state. A PCM cell may be put into one state or the other to indicate a stored value. By providing a potential across the PCM cell, the state of the PCM cell can be determined by measuring current flowing through the PCM cell. A PCM cell has a much higher on-current than off-current.
One architecture in common use for memories is a NAND architecture. In a NAND architecture, two or more memory cells are coupled together into a string, with the individual cell control lines coupled to word lines. A NAND string may be coupled to a bit line at one end of the NAND string. A bit of a NAND string may be read by addressing one cell of the string and turning on all the other cells in the string. If the addressed memory cell is on, the on-current will flow through the string, but if the addressed memory cell is off, only an off-current, which is much lower than the on-current will flow.
Some memory devices may create stacks of memory cells in a three-dimensional array. A stack of memory cells may include any number of memory cells. In some cases, the cells of a stack may be coupled into a NAND string but in other cases, multiple layers of control lines may be used to address cells of layer individually.